1. Field of the Invention
The invention relates to a method for coordinate measurement on workpieces using a coordinate measuring machine (CMM) in which the measured data are corrected by means of stored correction values.
2. Relevant Prior Art
To increase the measuring accuracy of CMMs, the measured data, i.e. the coordinate values, supplied by the measuring scales in the machine's axes have been provided for some time now with mathematical corrections. Normally, corrections are taken into account which are a result of the static guideway deviations of the CMM measuring carriages from a straight and orthogonal coordinate system, these deviations being derived according to the so-called "rigid model". Such correction procedures are described, for example in the paper "Numerical Error Correction of a Coordinate Measuring Machine" by K. Busch, H. Kunzmann and F. Waldele in the Proceedings of the International Symposium on Metrology for Quality Control in Production, Tokyo 1984, pp. 284-288, and in U.S. Pat. No. 4,819,195.
It is also a known procedure for tactile coordinate measuring machines to determine the amount of probe bending during contact with the workpiece being measured and to take this amount into account in the subsequent coordinate measurement. Such a procedure is described, for example, in paper "Korrektur der Taststiftbiegung bei Messung mit Mehrkoordinaten-Me.beta.geraten" (Correction of Probe Bending When Measuring with Coordinate Measuring Machines) by A. Weckenmann, G. Goch and H. D. Springborn, in Feinwerktechnik und Me.beta.technik 87 (1979) 1, pp. 5-9 and in the paper "Messende Taster mit mehreren Freiheitsgraden" (Measuring Probes with Several Degrees of Freedom) by W. Lotze in Technische Rundschau, issue 50 (1992) pp. 20-25.
In this procedure, the bending tensor of the respective probe is defined by probing a calibration sphere several times using different measuring forces. From this, the correction values for the different probes used in measuring a workpiece are calculated and stored, and taken into account in the coordinate measurements of unknown workpieces.
Furthermore, it is known to correct the dynamic deviations of the measuring slide of a CMM, i.e., the errors which occur due to the fact that the CMM's probe head mounted on the end of the horizontal arm is not at rest, but vibrates about its neutral position as a result of error influences such as drive acceleration, for example. Such a correction procedure is described, for example, in U.S. Pat. No. 4,333,238. Such dynamic correction methods operate with sensors which are mounted in the vicinity of the vibrating probe head and which generate signals representing the course of the spurious oscillation at the moment the probing contact is made.
Finally, error influences in an elastically modeled coordinate measuring machine have been examined in the two thesis papers of Eindhoven University by J. W. U. C. Teeuwsen dated Jan. 27, 1989, see chapter 3, and by H. Soons dated Jun. 16, 1993, see pp. 85-91. However, the authors take into consideration as error sources only the shift of mass due to the mobile measuring slides, ignoring the influence of the measuring or probing forces occurring between the workpiece and the coordinate measuring machine.
As coordinate measuring machines are being built lighter and lighter to achieve higher measuring speeds, elastic distortion of the measuring slides is increasingly gaining importance due to the probing force between the probe and the workpiece. This distortion depends on the extension of the CMM measuring slide and therefore causes deviations of the measured position data of different magnitudes as a function of the position of the probe head within the CMM measuring range.